Many modem machines are designed to operate in relatively rugged environments where maintaining traction can be challenging. Off-highway trucks, tractors, loaders and graders in particular are often required to perform in poor underfoot conditions. When traveling across slick mud or ice, loose rock, etc., or when pulling a load or pushing a material pile, the rim pull of one or more wheels of the machine can be sufficient to cause wheel slip.
Most off-highway machines have a differential coupled with one or more axles so that the wheels may rotate independently of one another. While such differentials facilitate turning of the machine and thereby reduce wear on certain components, they allow torque to be disproportionately or exclusively provided to a slipping wheel. Once a wheel begins to spin, the wheel at the opposite end of the axle may not be powered at all, at best reducing productivity and, at worst, stranding the machine. Thus, while the use of differentials is certainly advantageous, they can introduce their own set of challenges.
One method of addressing wheel slip problems in wheeled machines having differentials is to provide a selective braking mechanism which applies a brake to a spinning wheel, and thereby allows torque to be returned to the opposite wheel of a given axle. While selective braking approaches have worked well, they tend to cause undesirable wear of the brake components, and require the use of relatively complex control systems to sense wheel slip and selectively brake an individual spinning wheel. Moreover, using the machine brakes to reduce wheel slip inherently generates heat, which can prevent the use of brakes for traction control over long periods of time.
Another approach engineers have taken to wheel slip problems has been to provide a selective differential locking mechanism that allows an operator to rotatably couple the wheels of an axle together. Rather than one wheel spinning while the other sits idle, when the differential is locked the wheels rotate together, continuing to provide motive power to the machine even if one wheel encounters a slick or loose surface. Conventional locking differentials include a clutch that may be engaged to frictionally couple each half of the subject axle together. An operator control button, lever or similar device is typically positioned in the operator cabin, such that he or she may engage the differential lock when a wheel begins to slip. While certain of the known locking differential designs have met with technical and commercial success, such designs have various shortcomings.
For instance, because lockable differentials are typically manually operated, the operator can be distracted from other controls in the cab by trying to appropriately lock or unlock the differentials. The operator will also typically not manually engage the lockable differential until the wheels are slipping substantially, resulting in undue wear on the tires. Similarly, the operator often may not unlock the differential at an appropriate time, resulting in wear and tear not only on the tires, but also on other components of the machine drive train.
In an attempt to address the shortcomings of certain manual designs, a variety of designs and processes have been introduced which are directed to automating certain aspects of differential locking control. One known design is described in U.S. Pat. No. 4,549,448 to Kittle. Kittle discloses an agricultural tractor, and discusses a differential lock control system for the tractor which includes a hydraulically-operated differential lock coupled with a solenoid valve. The solenoid valve may be energized to unlock the differential in response to sensed vehicle speed and brake application. The differential may be re-locked automatically based on other vehicle parameters such as steering pressure, wheel slip and draft force. In Kittle, locking and unlocking of the differential is thus controlled based on separate parameters.
While Kittle offers a design and process suitable for certain operating environments and certain machine types, there is always room for improvement. Kittle's use of separate operating parameters to trigger differential locking versus unlocking requires a relatively complex control and monitoring system. Moreover, in at least certain applications, Kittle's design is incapable of optimal operating efficiency. The differential may be unlocked responsive to a particular operating parameter, but not re-locked even where that parameter returns to a value or range where locking is appropriate.
The present disclosure is directed to one or more of the shortcomings or problems set forth above.